This invention relates to testing and characterization of microwave high power transistors and amplifiers (device under test, DUT); the test system disclosed here allows for testing a DUT through acquisition and processing of its RF characteristics, typically gain, output power, efficiency, intermodulation etc., under precise source and load impedance conditions at the fundamental and harmonic frequencies (see ref. 1); reflection factors Γ up to and exceeding |Γ|=1 using a single fundamental frequency signal source and a single power amplifier for both input and output power injection are possible.